Granular media filters and demineralizers, as used extensively for the purification of water, typically comprise a carefully graded particulate media filter bed of sand, anthracite, ion exchange resins etc., which filter bed is supported above a floor positioned in a filter tank so as to divide the tank into an inlet chamber thereabove and an outlet chamber therebelow. In use, as the water to be treated is passed downwardly through the filter bed, the suspended solids and/or dissolved impurities are removed and retained within the granular media. Periodically, it is necessary to remove the suspended solids collected in the filter bed by backwashing the filter bed so as to preclude the filter bed from clogging and ceasing to properly function or to regenerate an exhausted ion exchange resin bed. Backwashing is conventionally achieved by passing backwash water upwardly through the filter media, preferably proceeded by or accompanied by a backwash gas such as air.
The collection of filtrate and the distribution of backwash air and/or water is commonly achieved by specially designed strainers or underdrain distributors which are received through openings in the floor of the filter tank and connect with pipes or a plenum space beneath the floor. It is particularly advantageous that the underdrain distributors fit closely to the floor and that they uniformly distribute the backwash air and/or water over the entire area of the filter bed. Examples of prior art stainers and underdrain distributors are described in U.S. Pat. Nos. 3,730,348, 4,214,992 and 4,707,237.
While the heretofore proposed strainers and underdrain distributors have been effective in directing backwash air and/or water upwardly through the filter bed, they have resulted in a "jetting" action causing the deposition of the lighter upper layer of the filter media bed to work its way down around the strainers resulting in high pressure drops through the strainers and possible media leakage during the service cycle. Such strainers have also required the use of a greater number of smaller strainers spaced a short distance apart to avoid dead spots which creates distribution problems and increases the cost.